For data transmission on a standard voice grade telephone line a modem is employed to modulate data in a form suitable for transmission along the telephone line, and to demodulate data received from the telephone line to reproduce the data transmitted from a remote site. The modem requires a power supply for energizing its electronic circuits, usually an AC power outlet or a battery source.
Providing the power for a modem is generally not a problem with computers which themselves operate from an AC power outlet. However, for battery operated portable computers, providing the power for a modem can be a significant drain on the limited power stored in the battery. This is particularly true if the modem is expected to monitor incoming calls over a long time period.
The ability to detect an incoming call and notify the host computer is a vital link to the usability of a particular modem/computer combination in providing electronic mail and remote database accessing functions. Any networking, database showing or database updating system that relies on automatic dialing and answering between computers must have the ability to detect incoming calls, referred to as ring detection capability.
Another vital link is the capability to run standard application software on the host computer transparently and independently without additional control from the host computer.
Modems in the past have required power to be on in order to detect and notify the host that an incoming call is being received. In a battery-powered computer, this is a critical waste of power which makes the system unacceptable. For example, if a computer consumed 1.3 mW in its "standby" mode while awaiting incoming data, but used a modem which required 225 mW of power to monitor and detect incoming calls, a battery which could provide one year "standby" power for the computer alone would provide only 40 hours of "standby" power for the computer and the modem together. Thus adding a high power consumption modem to a portable computer may negate the carefully designed power saving features incorporated in the computer.
There have been several alternative approaches proposed to provide ring detection and notification capability in the past, generally requiring that the modems be powered.
Modems commonly used with computers cannot remove power from selective circuitry yet still be able to monitor incoming calls. Thus the host computer will not be notified of the occurrence of an incoming call unless full power is applied to the modem.
An alternative approach is to draw the operating power for the modem directly from the telephone lines rather than from the computer. Although this type of system does not deplete the computer's battery power, it suffers from other drawbacks. Federal regulations strictly limit the power that can be drawn from the telephone lines. This severely restricts the functions which a telephone line powered modem can perform. Typically, such modems are limited to low transmission rates in the range of 300 baud. Also, the power limitation prohibits implementation of the command set necessary to function as an "intelligent" modem.
An additional approach is to provide a control signal from the computer to the power supply section of the modem. When the modem is required, the computer provides the control signal to apply power to the modem. When the modem is no longer needed, the computer removes the control signal to remove power. However, the time required for the computer to apply power to the modem can cause the execution of a software application on the computer to be unnecessarily delayed. Also, there are circumstances when the computer does not know that power should be applied to the modem, such as the occurrence of incoming calls.
It would be desirable for a modem to be able to enter a reduced power mode and be able to detect incoming calls and computer commands while in this mode. It would also be desirable for a modem to draw power from the computer to allow transmission at all baud rates and to allow the modem to function as an "intelligent" modem. It would also be desirable for a modem to be immediately accessible without having to wait for a control signal from the computer. It would also be desirable for a modem to be able to enter or exit from a reduced power mode in a manner transparent to the computer.